Heat the water and butter slowly in a pan until the butter has melted.
Remove from heat, quickly add all the flour and beat until the mixture leaves the side of the pan.
Add the egg a little at a time, beating thoroughly between each addition.
Beat in the cheese.
Spoon the mixture into a piping bag fitted with a 1 cm (1/2 inch) plain nozzle and pipe tiny mounds onto a dampened baking sheet, spacing them well apart.
Bake in a preheated moderately hot oven, 200°C (400°F), Gas Mark 6, for 12 to 15 minutes, until crisp and golden.
Make a small slit in the side of each puff.
Mash the tuna with the mayonnaise and spoon a little into each.
Serve warm, garnished with parsley.